Reaction sintering of β-tricalcium phosphates and their mechanical properties

Various kinds of calcium oxides, carbonates and phosphates were used as the raw materials, and β-TCP ceramics was fabricated by reaction sintering at 1100 °C, and the sinterability, the reaction sintering behavior and mechanical properties of reaction-sintered β-TCP were investigated. Reaction-sintered bodies using CaHPO₄ + HAp consisted of single β-TCP phase, and bulk density and bending strength increased with extending sintering time. On the contrary, normal-sintered β-TCP synthesized using CaHPO₄ + HAp did not change in bulk density and bending strength with extending sintering time. Reaction-sintered body using CaHPO₄ + HAp as the raw materials showed higher bulk density and bending strength than normal sintered β-TCP.     

Sintering behavior and growth mechanism of β-TCP in nanocrystalline hydroxyapatite synthesized by mechanical alloying

Nanocrystalline carbonated HAp powder has been synthesized successfully within 2 h by mechanical alloying the stoichiometric mixture of CaCO₃, CaHPO₄·2H₂O at room temperature under open air. To observe the sintering behavior of HAp the as-milled sample is sintered at different temperatures. The amorphous HAp phase (~14 vol%) in as-synthesized sample transforms completely to crystalline HAp after sintering at 700 °C and after sintering the sample at 800 °C, the crystalline HAp partially transforms to β-TCP phase. Presence of low content of β-TCP phase in HAp powder could be useful in artificial hard tissue applications. Increase in sintering temperature up to 1000 °C results in enhancement of decomposition rate of HAp into β-TCP phase. Microstructure characterization in terms of lattice imperfections and relative phase abundances in non-sintered and all sintered samples are made both by analyzing the respective XRD patterns using Rietveld's structure refinement method as well as TEM images. The growth mechanism of β-TCP from crystalline HAp phase has been proposed based on structure and microstructure characterizations of sintered samples.     

Processing and mechanical properties of hydroxyapatite pieces obtained by the gelcasting method

The aim of this work was to prepare hydroxyapatite (HA) pieces, by the gelcasting method, with appropriate mechanical properties for using as bone implants. The mechanical properties of the obtained pieces were evaluated. The influence of the solid content on the rheological behaviour of the slurries and, in turn, on the mechanical properties, was investigated. Pieces were prepared by using slurries containing 55 and 60 vol.% of an HA calcined at 1200 °C and 47 vol.% of HA calcined at 1100 °C. HA powders calcined at lower temperature than 1100 °C were not suitable for producing concentrated slurries. The raw and calcined HA were characterised by XRD, FTIR, X-ray fluorescence and elemental analysis. The pieces sintered at different temperatures and times were characterised by XRD, and the sintering treatment applied to the specimens for mechanical testing (1300 °C for 24 h) was selected taking into account the phases present after sintering.The mechanical properties of the green bodies were higher than obtained by other methods and the ones of the final pieces were comparable to those obtained by other techniques. The results indicated that slurries with a solid content higher than a limit value (for which limited contraction occurs) must be used and that the rheological properties of the slurries play an important role in the mechanical properties of the resulting pieces.     

Application of hydrophilic ionic liquid treatment to the morphological observations of hydrated porous ceramic green bodies

In this study, a simple and convenient method for observing the surface morphology of hydrated porous ceramic green bodies is proposed. The porous hyrdoxyapatite (HAp) green body was prepared by a gelcasting process and was dried in a humid chamber from 90 to 50% relative humidity at 25 °C before subsequent treatment with a hydrophilic ionic liquid (IL). The surface morphology of the IL-treated porous HAp green body was observed using FE-SEM. The results showed that the pore morphology and microstructure of the HAp green body was readily observable without evidence of charging. The as-prepared sample showed pores approximately 300–600 μm in diameter, which gradually contracted to approximately 200–400 μm upon drying in the humid chamber. Following sintering at 1000 °C, the pores had further contracted to approximately 100–300 μm. The IL binds with the surrounding water to prevent the sample from drying in vacuum and acts as a conductive media, allowing the HAp ceramics to be observed in the electron microscope. In comparison to the micro-focused X-ray CT analysis, the fine pore structure (less than 100 μm) could only be observed using FE-SEM when the porous body had also been subjected to the IL treatment.     

Preparation and properties of AlN ceramic suspension for non-aqueous gel casting

A non-aqueous gel casting process based on the mixed solvent (ethanol and polyethylene glycol) and low-toxicity N,N-dimethylacrylamide (DMAA) was developed for an aluminum nitride (AlN) ceramic. In the present work, rheological properties of non-aqueous concentrated AlN suspensions were investigated in the presence of mixed solvent, dispersant, milling time, monomer and solid loading, in order to screen for the most suitable experimental conditions to obtain a good rheological behavior for gel casting. The results showed that the 50 vol% slurry with 0.2 wt% dispersant concentration, 2 h milling time, 6 wt% -monomer content, and a solvent ratio of 3:1, can meet the requirements for the casting process of AlN ceramic slurries. After being dried at 100 °C for 1 h, the optimum bulk density and maximum flexural strength of the AlN green bodies were as high as 1.97 g/cm³ and 18.68 MPa, respectively. SEM photographs revealed that the green body had a relative uniform microstructure when the solid loading was 50 vol%. The shrinkage and deformation of shaped sintered bodies prepared through gel casting were small after sintering. The sintering shrinkage, apparent porosity, bulk density and flexural strength were 14.8%, 0.22%, 3.21 g/cm³ and 310 MPa, respectively.     

Characteristic reaction processes in the system brushite–NaOH solution

The reaction of brushite (CaHPO₄·2H₂O, DCPD) with NaOH solutions ranging in concentration from 0 to 40 mass% were investigated at 25 °C. At low NaOH concentrations below 20 mass%, hydroxyapatite (HAp) formed directly through the reaction (1) DCPD → HAp. At middle concentrations of 20–27 mass%, DCPD changed once into Ca(OH)₂ and then into HAp, i.e., through the consecutive reaction (2) DCPD → Ca(OH)₂ → HAp. At high concentrations above 27 mass%, the consecutive reaction (3) DCPD → HAp → Ca(OH)₂ was concluded. The boundary concentrations, i.e., singular points, between the first/second and the second/third reactions were estimated to be ca. 20 and 27 mass%, respectively. HAp particles obtained were platelet aggregates of very fine HAp microcrystals. The platelet form was compatible to the original platelet DCPD form. HAp samples prepared in this reaction system were structurally low crystalline and compositionally stoichiometric, and had a feature of large heating weight losses below 400 °C.     

Preparation of dense β-CaSiO3 ceramic with high mechanical strength and HAp formation ability in simulated body fluid

In present study, dense CaSiO₃ (CS) ceramics have been fabricated through spark plasma sintering (SPS) technique using β-CS powder prepared by chemical precipitation method. The β-CS ceramic sintered at 950 °C has a relative density of about 95% and shows a fine microstructure with an average grain size of 0.6 μm, thus expresses good bending strength of about 294 MPa. The simulated body fluid (SBF) immersion tests show that the dense β-CS ceramic has a high hydroxyapatite (HAp) formation rate on its surface. The HAp layer formed on the CS ceramic surface has a granular structure consisting of silkworm-like HAp grains, and the thickness of HAp and Si-rich layer are 70 and 120 μm, respectively.     

Fabrication of structure-controlled hydroxyapatite/zirconia composite

The structure-controlled hydroxyapatite/zirconia (HAp/ZrO₂) composites were fabricated. At first, cylindrical hydroxyapatite (HAp) samples were prepared by the extrusion process, and then the extruded HAp cylindrical samples were coated with 3 mol% of Y₂O₃ partially stabilized ZrO₂ slurry, dried and aligned unidirectionally to form a composite bulk. The volume fraction of ZrO₂ in the HAp/ZrO₂ composite was estimated to be about 23 vol%. Bulk density and bending strength of the composites increased with sintering temperature. Fracture energy of HAp/ZrO₂ composite sintered at 1350 °C was approximately 1.6 times higher than that of monolithic HAp. Although the bending strength of HAp/ZrO₂ composite prepared in this study was relatively low, it exhibited high fracture energy than HAp monolithic and a non-brittle fracture behavior was obtained without using fiber as the reinforcement.     

Fabrication of sub-micrometer MgO transparent ceramics by spark plasma sintering

Transparent MgO ceramics were fabricated by spark plasma sintering (SPS) of the commercial MgO powder using LiF as the sintering additive. Effects of the additive amount and the SPS conditions (i.e., sintering temperature and heating rate) on the optical transparency and microstructure of the obtained MgO ceramics were investigated. The results showed that LiF facilitated rapid densification and grain growth. Thus, the MgO ceramics could be easily densified at a moderate temperature and under a low pressure. In addition, the transparency and microstructure of the MgO ceramics were found to be strongly dependent on the temperature and heating rate. For the MgO ceramics sintered at 900 °C for 5 min with the heating rate of 100 °C/min and the pressure of 30 MPa from the powders with 1 wt% LiF, the average in-line transmittance reached 85% in the range of 3 – 5 μm, and the average grain size is ∼0.7 μm.     

Natural and synthetic hydroxyapatite/zirconia composites: A comparative study

Zirconia precursor was precipitated in a HAp particles suspension using two HAp powders of natural origin and a synthetic powder. The first natural HAp was extracted from animal bones by treatment with hot NaOH solution and the second one by treatment under hydrothermal conditions with water.Hydrous zirconia was precipitated in the HAp suspension. Pressureless sintering was performed at 1000–1300 °C and hot pressing at 1050–1300 °C.It was found that zirconia additive promotes decomposition of both HAp of natural origin as well as the synthetic one. The most stable HAp was the one extracted from bovine bones by treatment with water in an autoclave. This reaction leads to the formation of β–TCP and the CaO–ZrO₂ solid solution.The hot pressed composites show essentially higher strength and fracture toughness as compared to the pure hydroxyapatite polycrystals.     

Pressureless sintered silicon carbide tailored with aluminium nitride sintering agent

This study reports the influence of aluminium nitride on the pressureless sintering of cubic phase silicon carbide nanoparticles (β-SiC). Pressureless sintering was achieved at 2000 °C for 5 min with the additions of boron carbide together with carbon of 1 wt% and 6 wt%, respectively, and a content of aluminium nitride between 0 and 10 wt%. Sintered samples present relative densities higher than 92%. The sintered microstructure was found to be greatly modified by the introduction of aluminium nitride, which reflects the influence of nitrogen on the β-SiC to α-SiC transformation. The toughness of sintered sample was not modified by AlN incorporation and is relatively low (around 2.5 MPa m^1/2). Materials exhibited transgranular fracture mode, indicating a strong bonding between SiC grains.     

Rapid and uniform in-situ solidification of alumina suspension via a non-contamination DCC-HVCI method using MgO sintering additive as coagulating agent

A novel rapid, uniform and non-contamination in-situ solidification method for alumina suspension by DCC-HVCI method using MgO sintering additive as coagulating agent was reported. MgO was used to release Mg²⁺ in suspensions via reaction with acetic acid generated from glycerol diacetate (GDA) at elevated temperature as well as to improve density and suppress grain growth of alumina ceramics during sintering. Influence of adding 0.7 wt% MgO with 2.0 vol% GDA in alumina suspension on coagulation process and properties of green bodies and sintered samples were investigated. It was indicated that the controlled coagulation of the suspension could be achieved after treating at 70 °C for 10 min. Homogeneous composition distribution of Mg element in EDS result indicated the uniform solidification of suspensions. Compressive strength of wet-coagulated bodies is 2.09±0.25 MPa. Dense alumina ceramics with relative density of 99.2% and flexural strength of 354±16 MPa sintered at 1650 °C for 4 h present homogeneous microstructure. The result indicated that the novel DCC-HVCI method via a sintering additive reaction with no contamination, short coagulation time and uniform in-situ solidification is a promising colloidal forming method for preparing high-performance ceramic components with complex shape.     

The densification of zeolite/apatite composites using a pulse electric current sintering method: A long-term assurance material for the disposal of radioactive waste

Pulse electric current sintering (PECS) method was applied to the fabrication of zeolite, hydroxyapatite (HAp) and fluorapatite (FAp) sintered bodies that should be long-term assurance materials for the disposal of radioactive waste. The weight ratio of zeolite and apatite was ca. 3/7. Zeolite powder evenly covered with HAp thin layers prepared by a hydrothermal method and spherical FAp powder by spray dryer were employed for the PECS; the sample was sintered at 900 °C for 10 min at a rate of 50 °C/min under a uniaxial pressure of 50 MPa and then cooled to 600 °C at 5 °C/min in vacuum. The powder X-ray diffractions indicated that the structure of zeolite changed to the amorphous. The zeolite powder was well dispersed in FAp matrix as the results of element mapping analyses by energy dispersive X-ray spectrometer. The observations by a scanning transmission electron microscope indicated that amorphous zeolites were covered with needle-like HAp crystals of which layer completely coupled with sintered FAp grains. The HAp thin layers thus play an important role for improving the affinity between FAp matrix and the zeolite. The microhardness and three-point bending strength of the sintered bodies were also elucidated by a dynamic-ultra microhardness tester and a universal tester, respectively.     

Tribo-mechanical characterization of spark plasma sintered chopped carbon fibre reinforced silicon carbide composites

Short carbon fibre (C_f) reinforced silicon carbide (SiC) composites with 7.5 wt% alumina (Al₂O₃) as sintering additive were fabricated using spark plasma sintering (SPS). Three different C_f concentrations i.e. 10, 20 and 30 wt% were used to fabricate the composites. With increasing C_f content from 0 to 20 wt%, micro-hardness of the composites decreased ~28% and fracture toughness (K_IC) increased significantly. The short C_f in the matrix facilitated enhanced fracture energy dissipation by the processes of crack deflection and bridging at C_f/SiC interface, fibre debonding and pullout. Thus, 20 wt% C_f/SiC composite showed >40% higher K_IC over monolithic SiC (K_IC≈4.51 MPa m^0.5). Tribological tests in dry condition against Al₂O₃ ball showed slight improvement in wear resistance but significantly reduced friction coefficient (COF, μ) with increasing C_f content in the composites. The composite containing 30 wt% C_f showed the lowest COF.     

Fabrication of yttria-stabilized-zirconia thick coatings via slurry process with pressure infiltration

Yttria-stabilized-zirconia (YSZ) coatings with thicknesses up to 420 μm have been prepared using a novel slurry process with pressure infiltration. Binary-sized particle slurries (binary-slurries), composed of nano-particle slurry (nano-slurry) and micro-sized preformed particles, were cast on metal substrates to form coatings. After sintering at 1150 °C for 1 h, preformed particles were cemented with nano-particles to form a porous YSZ coating. Subsequently, the nano-slurry was infiltrated into the porous coatings under pressure. The infiltrated nano-slurry filled the pores, and was sintered together with the porous coating, resulting in an increase in both density and mechanical properties of the coating. After 5–6 infiltration cycles, the coating reached 82% theoretic density and micro-hardness of 3.7 GPa. Such coatings could be used as thermal barrier coatings for high temperature applications.     

Sintering behaviour and properties of magnesium orthosilicate-hydroxyapatite ceramic

The effect of pressureless sintering on the properties of magnesium orthosilicate-hydroxyapatite (MO-HA) ceramic has been studied. The amount of MO composition in the green body was varied from 10 wt% to 50 wt% through mechanical ball milling and was subsequently sintered at varying temperatures in air atmosphere from 1000 °C to 1300 °C for 2 h. The magnesium orthosilicate phase was stable during sintering but the hydroxyapatite phase decomposed to tricalcium phosphate. The MO-HA composites generally exhibited lower mechanical properties across all the investigated composition. Nevertheless, a high fracture toughness of 2.5 MPam^1/2 was recorded for sintered body that contained 20 wt% MO. This finding indicates the potential of this ceramic composite to be used for biomedical applications.     

Effects of glass additions on the microstructure and dielectric properties of barium strontium titanate (BST) ceramics

Commercial glass frits (lead borosilicate glasses) were employed as the sintering aids to reduce the sintering temperatures of BST ceramics. The effects of the glass content and the sintering temperature on the microstructures, dielectric properties and tunabilities of BST ceramics have been investigated. Densification of BST ceramics of 5 wt% glass content becomes significant from sintering temperature of 1000 °C. The glass content shows a strong influence on the Curie temperature T_c, permittivity and the diffuse transition. X-ray results show all BST ceramics exhibit a perovskite structure and also the formation of a secondary phase, Ba₂TiSi₂O₈. The shift of BST diffraction peaks towards higher angle with increasing the glass content indicates the substitution of Pb²⁺ in Ba²⁺ site, which mainly accounts for the diffuse transition observed in these BST ceramics. BST ceramics with 10 wt% glass additives possess the highest tunability at all four sintering temperatures. A tunability of 12.2% at a bias field of 1 kV/mm was achieved for BST ceramics with 10 wt% glass content sintered at 900 °C.     

Aqueous tape casting process for SiC

The condition for the preparation of stable SiC slurries by aqueous tape casting was identified. To acquire stable uniform slurries, the influences of dispersant, solid loading, sintering additives, binder and plasticizer on the rheological properties and viscosity were investigated. The conditions for preparing stable SiC slurries were studied and optimized. After tape casting and drying, the green SiC sheets showed smooth surface and homogeneous microstructure. The SiC ceramic can be densified to 98.89% after hot-pressing at 1850 °C (at 25 MPa in Ar for 30 min). The flexural strength, hardness, and toughness are 779.5 ± 39.2 MPa, 21.51 ± 0.70 GPa and 5.54 ± 0.26 MPa m^1/2, respectively. SEM shows a fine microstructure with few pores in the sintered samples. The fracture surface exhibited predominantly intergranular fracture type.     

Sintering behavior of waste serpentine from abdasht chromite mines Abdasht chromite mines and kaolin blends

The aim of this study is investigating on sintering behavior of Abdasht waste serpentine and kaolin blends. According to this, three formulations of dry milled waste serpentine with 25%, 50% and 75% high grade kaolin were wet milled. The slurries were then dried, sieved and uniaxially pressed at 100 MPa and fired for 2 h soaking time at temperatures between 1100 and 1400 °C. Sintered samples were investigated by bulk density, apparent porosity, water absorption, linear shrinkage and phase changes with raising temperatures in order to characterize their sintering process. It was revealed that all samples were starting to melt at 1350 °C and the sintering was completed for all specimens at 1300 °C. The only phases of fully sintered samples were cordierite and enstatite. Cordierite concentration, however, increased with enhancing kaolin percentage in composition. The results of this study can introduce Abdasht waste serpentine as magnesium silicate source into the ceramic industries and may help to solve environmental problems caused by several million tones wastes in Abdasht chromite mines.     

Fabrication of transparent yttria ceramics by alcoholic slip-casting

Y₂O₃ transparent ceramics were prepared from alcoholic slurries of Y₂O₃ nanopowders via a slip-casting method to avoid the hydrolysis issue. Polyvinyl pyrrolidone (PVP), polyethylene glycol (PEG) and polyethylenimine (PEI) were used as dispersants to improve the rheological properties of the slurries. It was found that PEI is the most effective dispersant in ethanol. The adsorbed amount of PEI was evaluated by infrared absorption and rheology measurements. Y₂O₃ slurry with a solid loading of 20.8 vol% and a viscosity of <0.1 Pa s at the shear rate of 10 s⁻¹ was obtained using 1.5 wt% PEI. The slurry yielded a homogeneous green body, and finally resulted in a high-quality Y₂O₃ ceramic with the in-line transmittance of 80% at 800 nm.